The Transition Zone: Slabs’ Purgatory

CIDER, 2006 - Group A

Garrett Leahy, Ved Lekic, Urska Manners, Christine Reif, Joost van Summeren, Tai-Lin Tseng,

Magali Billen, Wang-Ping Chen, Adam Dziewonski

Tonga Seismicity

Predicted Slab Positions

Degree 45 and 24 spherical harmonic expansions of locations of slabs based on plate history reconstructions assuming no stagnation in transition zone.

Tomographic Models

Harvard Berkeley

Preliminary Conclusions

• Tomography reveals larger fast regions in the western Pacific transition zone.

• Deep earthquake stress axes show evidence of resistance to crossing the 660 km discontinuity.

• Structure below and above 660 km discontinuity has different spectral character.

• Implication: slabs stagnate in the transition zone for some length of time.

A Simple Force Balance for slabs in the Transition Zone

Fb =∫ gdxdz

x

z

Constraints on and Clapeyron slopes

• Density contrasts– Seismic constraints– Lab experiments on mantle minerals/rocks– Lattice dynamics simulation

• Clapeyron slopes– Lab experiments on phase transformation– Calorimatric Calculations

Summary Phase Transition Data

  Seismic Constrains Calculations (Pyrolite)

Simulations (MgSiO3)

410 5% to 6% About 3%  

660 7% to 9% 6% to 7% About 8%

  Lab Experiments Calorimatric Calculation

dP/dT 410 (Mpa/K) to 2.5 to 4

dP/dT 660 (Mpa/K) to Mw+Pv

–3 to –1 About -3

dP/dT 660 (Mpa/K) Pyrolite -0.5  

Density Contrast

Clapeyron Slope

For Clapeyron Slope of Olivine Polymorphs: Duffy, T., Synchrotron facilities and the study of the Earth's deep interior. Rep. Prog. Phys. 68 (2005) 1811-1859.

Slab Thermal AnomalyGaussian Cross-slabProfile Exponential

DecreaseIn PeakAnomaly

Max. SlabDepth: 1000 km

Max. SlabDepth: 500 km

Phase Transition Anomaly

Temperature AnomalyTransition Height (km)

410: = 3.0 MPa/K = 3-6%660: = -1.3 MPa/K = 7-9%

410: = 4.0 MPa/K = 4%

660: = -2 MPa/K

= 3%

Effect of Dip on Sum of ThermalAnd Phase Change Forces

0 10 ---Dip (degrees)-- 80 90

Tota

l For

ce (x

1012

N/m

)

16

12

8

4

0

Effect of Density Change at Phase Boundaries

Change in Density at 660 (%)

Cha

nge

in D

ensi

ty a

t 410

(%)

6 6.5 7 7.5 8 8.5 9

6

5.

4

3

Effect of Clapeyron Slope

Clapeyron Slope at 660 Mpa/KCla

peyr

on S

lope

at 4

10 M

pa/K

-3 -2 -1 -0.5

5

4

3

2.5

Effect of Shear Forces

Major slowing occurs upon entering lower mantle

Lower mantleviscosity greaterthan 1022 Pa scan strongly hinderSlab.

um=1019 Pastran = 1020 Pas

Metastable OlivineGrowth Rate:

G(T) = A*k*T*exp[-H/(RT)](1-exp[G/(RT)])

k=exp(10) Growth constant A = 1e-3 Extrapolation parameter for

low T in slab.

Depth of Metastable Olivine in Slabz ~v*ln(1-f)/(-2*S*G)

v Slab velocityS = 1/d Grain boundary Surface

Area/Volumef = 0.95 Volume fraction of

wadsleyite at completion of transformation.

Cooler Temperature strongly inhibits transformation.

What about a Metastable Olivine Wedge?

Conclusions• Buoyancy from temperature can be order of magnitude

larger than other forces.– Need dynamic model of temperature.

• Extra buoyancy from 410 phase change may be much larger than resisting buoyancy from 660.

• Shear forces beneath 660 may significantly hinder slab sinking into lower mantle.

• If phase parameters at 410 and 660 are comparable, then a moderately high viscosity in lower mantle can hinder slab.

• If metastable olivine exists, it can “easily” stop slabs in the transition zone, especially for large grain size (~ cms)